Stock-Based Compensation - Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Compensation expense:
Total stock-based compensation expense	$ 45,274	$ 65,705	$ 151,457	$ 77,602
Additional disclosures:
Unrecognized compensation expense	209,000		$ 209,000
Unrecognized compensation expense, recognition period			1 year
Research and development
Compensation expense:
Total stock-based compensation expense	21,860	28,730	$ 70,429	34,735
General and administrative
Compensation expense:
Total stock-based compensation expense	$ 23,414	$ 36,975	$ 81,028	$ 42,867